1. Field of the Invention
The present invention relates to a method utilized in a wireless communication and communication device thereof, and more particularly, to a method of handling an inter-RAT (Radio Access Technology) handover in a wireless communication system and communication device thereof.
2. Description of the Prior Art
A handover procedure can transfer a mobile device from a communication state to another communication state. The communication state change can correspond to a telecommunication system, a cell, or a frequency layer scale, and corresponding handover is known as an inter-radio access technology (RAT), an inter/intra-cell or an inter-frequency handover.
For the inter-RAT handover, an X RAT can use an X handover procedure to hand over a mobile device to a Y RAT by sending an X handover message including Y RAT configuration information. The mobile device configures itself to fit system requirements of the Y RAT according to the received Y RAT configuration information. Mostly, the Y RAT configuration information is formed as a message conforming to the specification of the Y RAT and includes mobility management, signaling connection, security, and capability information. Common RATs include UMTS (Universal Mobile Telecommunications System), GSM (Global System for Mobile communications), GERAN (GSM/EDEG radio access network) Iu mode system, CDMA (Code Division Multiple Access) 2000, and LTE (Long Term Evolution) systems.
In the LTE system, a “mobility from E-UTRA” procedure is used to hand over a user equipment (UE) from the LTE system to another RAT, such as the UMTS, or GSM, GERAN Iu mode. For a handover to UMTS system, the radio access network of the LTE system, E-UTRAN (Evolved UMTS radio access network), initiates the mobility from E-UTRA procedure for a UE in a RRC_CONNECTED mode by sending a MobilityFromEUTRACommand message including a “HANDOVER TO UTRAN COMMAND” message that is a handover message of the UMTS system.
Due to the pure packet switched (PS) feature of the LTE system, a CS (circuit switched) fallback handover is introduced to an EPS (Evolved Packet System), enabling the provisioning of voice and other CS-domain services by reuse of CS infrastructure of the UMTS or GSM system when the UE is served by E-UTRAN. A CS fallback enabled terminal, connected to E-UTRAN, may use the GERAN or UTRAN (UMTS radio access network) to establish one or more CS-domain services. The CS fallback handover is only available in case E-UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.
A successful handover requires proper cooperation of the UE, the source RAT, and the target RAT in the mobility, signaling connection, security, and capability aspects. In the signaling connection, a complete PS connection of the UMTS system is made up of two parts: an RRC (radio resource control) connection and an Iu connection part. The RRC connection is a connection between RRC layers of the UE and UTRAN, whereas the Iu connection is a connection between the UTRAN and the core network (CN) terminal, such as a SGSN (Serving GPRS Support Node) for the PS domain (i.e. Iu-PS connection) or a MSC (Mobile Switching Center) for the CS domain (i.e. Iu-CS connection).
In the abovementioned handovers from the LTE to the UMTS system, the Iu connection is certainly established, whereas the CS or PS signaling connection is established or not depended on a “RAB info” information element (IE) included in the HANDOVER TO UTRAN COMMAND message. The “RAB info” IE is used to identify a radio access bearer and has a “CN domain identity” IE that is used to indicate that a signaling connection is established for either PS or CS domain. In addition, when a HANDOVER TO UTRAN COMMAND message includes no “RAB info” IE, the UE determines that CS signaling connection is established.
On the standpoint of the UE, the whole PS connection is determined to be established when the UE receives a HANDOVER TO UTRAN COMMAND message that includes the “RAB info” IE and the “CN domain identity” IE indicates the PS domain, and to be unestablished when the received HANDOVER TO UTRAN COMMAND message does not include any “RAB info” IE or a received “CN domain identity” IE indicates the CS domain. An error in determining establishment of the PS connection is caused by the above UE actions.
Take an example. When a UE in an RRC idle mode of the LTE system makes a CS MO (Mobile Originating) call, a CS fallback handover is triggered, and the UE is thereby handed over from the E-UTRAN to UTRAN. During the handover, no “RAB Info” is included in the HANDOVER TO UTRAN COMMAND message because the UE in the RRC idle mode has no EPS RB (Radio Bearer) established for data transmission before the handover. Therefore, the UE determines that CS signaling connection is established. As a result, the UE sends an “RRC Initial Direct Transfer” message including a “CM Service Request” message to the UTRAN to establish the CS call. If PS data transmission (e.g. web browsing) is also triggered, the UE sends the “RRC Initial Direct Transfer” message including a service request to the UTRAN to establish a PS call, where the “RRC Initial Direct Transfer” message here triggers initiation/re-initiation of the whole PS connection. However, the Iu connection part of the PS connection has been established in UTRAN during handover procedure. In this situation, the UTRAN has to release the existing Iu connection when receiving the service request of the “RRC Initial Direct Transfer” message and then establishes a new Iu connection. This wastes system resource because the messages are required to be exchanged in UTRAN and CN for the unnecessary re-establishment of the Iu connection.
In the security configuration, the “CN domain identity” IE involves configuration of a START value, integrity, and ciphering. As well known, the UMTS system includes a STARTPS value for the PS domain and a STARTCS value for the CS domain, whereas only one START value is used in the LTE system. During the handover from the LTE to UMTS system, the UE needs to activate ciphering for all RBs (Radio Bearers) and SRBs (Signaling Radio Bearers) with the START value. The UE determines to use the STARTPS or the STARTCS value according to the domain indicated by the “CN domain identity” IE or use the STARTCS value when no “CN domain identity” IE is received. However, the UTRAN may not receive corresponding START value from the UE or the E-UTRAN during the handover. Different START values, irrespective of the domain type or the value, used between the UE and the UTRAN cause ciphering errors.
Take an example. When a UE in the RRC idle mode of the LTE system makes a CS MO call, a CS fallback is triggered, and the UE is handed over from E-UTRAN to UTRAN. In this case, no “CN domain identity” is included in the HANDOVER TO UTRAN COMMAND message, and also no configuration information of the START value is sent to the UTRAN. Therefore, the UE activates ciphering for all SRBs with the STARTCS value. However, the UTRAN may activate ciphering for all SRBs with the STARTPS value, causing ciphering errors.
For activation of integrity protection (IP), a security mode control procedure is initiated after the handover to the UMTS system instead of direct transmission of the IP configuration. This IP activating method is adopted in consideration of the HANDOVER TO UTRAN COMMAND message whose size is so critical for the GSM system that the IP information cannot be included during the handover from the GSM to the UMTS system. The integrity protection is started immediately after a handover by means of the security mode control procedure, causing a delay of the IP activation. Furthermore, the size of the HANDOVER TO UTRAN COMMAND message is not critical in the E-UTRAN since transmission bandwidth of the LTE system is increased much more than the GSM system. From the security aspect, the integrity protection associated with the handover from the LTE to UMTS system should always be started as soon as possible.
In the mobility management, CS call establishment involves UE location and the core network may not accept the establishment when the UE location information does not fit the system requirement. For example, a CS fallback handover is triggered when the UE makes a CS MO call in the E-UTRAN, and thereby the UE sends the “CM Service Request” message to establish the CS call. In case the MSC serving the 2G/3G target cell is different from the MSC that serves the UE while camping on the E-UTRAN, the MSC always rejects the CM service request if implicit location update is not performed for the UE. Moreover, the UE may not perform the location update after the MSC responds to the “CM Service Request” message, further delaying the CS call establishment.
In the capability requirement, a UE Capability Enquiry procedure is used for the E-UTRAN to obtain the capability of the UE. However, the related specifications do not specify whether to perform the UE Capability Enquiry procedure when the handover from the LTE system occurs and what capability information should be reported by the UE. In this situation, the target RAT may be unable to obtain capability information of the UE for the handover. Or, the UE may report insufficient capability information or capability information of a wrong target RAT through the UE Capability Enquiry procedure. Those cause signaling or connection errors after the handover.
In the prior art, the inter-RAT handover can fail or cause disconnection of a RRC connection when the capability, mobility, or security configuration/procedure for the handover is improperly performed as mentioned above.